Stellar Spectra

AST 112 Lecture 7

Stellar Spectra

• The interior of a star can be considered a “hot dense object” that emits a continuous spectrum.

• The interior is surrounded by a much cooler atmosphere.

• What type of spectrum (emission, absorption or continuous) would you expect to see from a star?

Stars show an absorption spectrum.

* Some stars show emission features as well.

Stellar Spectra

• Stellar spectra tend to resemble one of these seven spectra

Stellar Spectra

• Why do stars have different spectra?

Pioneers of Stellar Spectroscopy

1

2

3

4

5

6

7

The Old Classification

• First classified by strength of hydrogen lines

• Old Classification:A, B, C, D, …

The Old Classification

• Maybe not perfect?

7000 oF

10,000 oF

12,000 oF

15,000 oF

17,000 oF

30,000 oF

60,000 oF

The Harvard Classification

• More natural order followed (found by Annie Jump Cannon)

• This order follows temperature

• Everything’s great, right??

Hig

her T

empe

ratu

re

The Balmer Series

• These absorption lines are called the Balmer Series.

• They occur when the electron in hydrogen is in the first excited state and absorbs a photon

The Balmer Series

• So in order to see Balmer lines, some fraction of the hydrogen atoms must have their electrons in the first excited state.

• The more hydrogen atoms in the first excited state, the darker the absorption lines.

A Discrepancy

• Quantum Mechanics:

– H absorption lines should get stronger as temperature goes up

• The order of the old scheme looks right

– But temperature is wrong!

7000 oF

10,000 oF

12,000 oF

15,000 oF

17,000 oF

30,000 oF

60,000 oF

The graph does not agree with the spectra!

The Harvard Classification

• Saha: Electrons detach at these temperatures!

• If the electron of H is detached, absorption lines don’t happen

Multiply the top graphs to get the bottom graph.

This graph peaks at 10,000 K (17,000 oF).

At what temperature would you expect the darkest H absorption lines?

What happens to the H absorption linesabove or below thistemperature?

Combine Graphs

Graph of line darknesspeaks at 17,000 oF

“A” Star

17,000 oF

OBAFGKM

• OBAFGKM is correct!

• The physics of the absorption lines agrees completely.

• Spectral type is determined by temperature.

Structure of Matter vs. Temperature

Cooler:Molecules Form

Warmer:Molecules

Break

HOT:Atoms Ionized

Same atoms. Different interactions. Different spectra.

Stellar Spectra

We can do this for all of the elements. This is howthe line strength behaves (with temperature) for each element. And it agrees with stellar spectra.

Stellar Spectra

Decreasing temperature: O B A F G K M

So what are stars made of?

• Chemical composition does not vary much from star to star

• Cecilia Payne: – All stars made up of:

• About 75% Hydrogen• About 25% Helium• About 1-2% heavier

elements

Stellar Spectra

If stars have little variation in composition, states of elements determine spectrum.

Temperature determines the state of theelements, and therefore the spectrum.

Measuring Stellar Temperature (Spectra)

• M type stars: 5000 oF• O type stars: 70,000 oF

• Each spectral type subdivided by number– B0, B1, … , B9– Larger number, cooler star

• Sun is G2• Sirius is A1• Betelgeuse is M2

Measuring Stellar Temperatures

• Recall Stefan-Boltzmann Law and Wein’s Law

Brightness = sT4

Peak Wavelength depends on T

• When do these laws apply?

• Do they apply to stars?

Measuring Stellar Temperatures (Color)

• Rank the following stars according to temperature, hottest first:

– Yellow

– Red

– Blue

Measuring Stellar Temperatures (Color)

• Rank the following stars according to temperature, hottest first:

– Blue (Sirius: 16,500 oF)

– Yellow (Sun: 10,000 oF)

– Red (Betelgeuse: 5600 oF)